You and Engineer have never been exactly "friends". The only times you've really interacted with him was thanking him whenever you went through one of his teleporters. However, that wasn't really much of a problem.

Fast-forward a few days later...

INTRUDER ALERT! BLU SPIES IN THE BASE!

You've been ambushed! Pretty much your whole team is dead, and is still going to respawn. Right now, it's up to you and Engineer, and his buildings to defend the intelligence.

Rushing to the intel room, you find Engineer hitting his wrench against his sentry. He spots you, and... he immediately picks up his shotgun and aims it at you. ... wait, what?

"Not so fast, mister. Prove you're not a Spy, or else you're gettin' a bullet in your head." he said, smiling. Not even smirking, smiling.

You put your hands up. You were kind of expecting this, so you already had an idea on how you were gonna prove him you weren't a Spy. Just tell him to ask you a question that only you could ans-... oh wait... still not friends. Uh...

"So? What's it gonna be?"